Rondos are characterized by the alternation of refrains and episodes. Each return of the refrain is in the tonic key, and presents the same melodic/motivic material, while episodes contrast melodically and tonally.

Refrains are generally simple, tight-knit themes that end with a PAC in the tonic key. Thematically, a rondo's refrain is most commonly constructed in one of two ways: (1) as a simple (usually tight-knit, often periodic) theme type, or (2) as a rounded binary.

If the refrain is a rounded binary, it is common for it to be abridged when returning later in the movement.

Episodes, by contrast, are usually in contrasting keys and feature contrasting melodic/motivic material. Though not always the case, it is most common for the episodes that contrast with refrains to be more complex in construction than refrains. Dramatizing the return to the refrain is an important part of the rondo aesthetic, and the thematic and harmonic complexity associated with rondo episodes forms an important part of that goal.

We distinguish between two types of episodic material: (1) interior themes, and (2) second-theme complexes. Because of the greater complexity, these two types of refrain are explored in more detail below.

Interior Theme

Interior themes are the simplest type of episode. Thematically and tonally, they resemble a Minuet's Trio.

Tonality: An interior theme contrasts modally or tonally with the refrain. Much of the time, the interior theme is "minore," meaning that the tonic does not change from refrain to episode, but the mode does. (In a minor-key rondo, the first episode might be "maggiore.")

Formal structure: An interior theme is usually a rounded binary, or one of the other binary types. Sometimes, the binary organization is shown with repeat signs in the score, though occasionally the repeats are "written out." Though by no means obligatory, an interior theme can be followed by a retransition that leads to the home key's dominant.

Complications: The primary complication for analysis is that interior themes are often left incomplete formally. Be on the look out for these two deviations;

1. Following a contrasting middle section, the recapitulation may not return. Thus, such an episode might be structured as AAB followed by a retransition.
2. The recapitulation may return but not lead to a PAC, instead merging into a retransition section.

Second-Theme Complex

A second-theme complex resembles—but is not always identical to—the TR, S, CL, and RT zones of the classical sonata. In a major key, the episode will modulate to the dominant (V). In a minor key, the major mediant (III) is more common.

Formal structure: We will outline the prototypical form first, though as indicated below, you must approach this section flexibly. Following the first refrain's close, a transition phrase (TR) begins the episode by modulating to a subordinate key through a pivot-chord modulation. The secondary theme (S) follows, confirming the new key with a PAC. Then, closing (CL) is heard and merges into a retransition (RT) that leads back to the refrain.

Complications: Though it resembles features of a sonata exposition, a rondo's second-theme complex engages many more possibilities. TR and CL may not appear, for example, and the second-theme complex may not have a PAC in the subordinate key. Thematic function is much looser here than in the refrain. Here are the primary complications:

1. TR merges => into S: Following the PAC that ended the refrain, a TR phrase begins and initiates a modulation. However, after modulating the same phrase confirms the subordinate key with a PAC. In this situation, the single phrase is understood to express both TR and S function. This is very much like the TR=>S merger that we find in the exposition of a Minuet/Trio form.
2. No TR appear: Following the PAC that ended the refrain, S appears with no intervening TR. In this case, a direct modulation has occurred.
3. S does not lead to a PAC, but merges => into a retransition (RT): Typically, the episodes S theme will lead to a PAC that confirms the subordinate key. In this case, the cadence never materializes. Instead, the music more or less seamlessly becomes a retransition, S=>RT.

Analytical Tips:

Cadential structure is absolutely necessary to determining a theme's function. This is even more the case with the rondo's second-theme complex. Here are some analytical tips, oriented around cadential structure, that you should use to guide your analysis.

1. S: themes begin in a subordinate key, end with a PAC in a subordinate key, or both. That is, a phrase that ends with a PAC in the subordinate key expresses S function even if the phrase did not begin in the subordinate key, and vice versa.
2. TR: function is given to phrases that modulate. When TR doesn't merge with S, it usually ends with a HC in the subordinate key.
3. CL: function follows a PAC in the subordinate key.
4. RT: usually contains a I:HC, optionally followed by "standing on the dominant."